Emoji Indexing and Image Replacement

This PR includes my first version of the gem as used for my http://polarb.com/ app to allow replacing unicode Emoji characters with image links from the image library in this gem.

* `64x64` emoji image library from phantom_open_emoji
* `Emoji::Index` utility loading phantom_open_emoji metadata
* `Emoji.replace_unicode_moji_with_images` API for injecting IMG tags with emoji images into a string
* `html_safe` and HTML escaping support for safety.
* README/Usage info on basic manual configuration